Electrochemistry of carbon monoxide on platinum single-crystal surfaces

Date

2008-08

Journal Title

Journal ISSN

Volume Title

Publisher

Texas Tech University

Abstract

Improving understanding of carbon monoxide (CO) adsorption and electrochemical oxidation on Pt type materials is important for the development of low temperature fuel cells and electro-synthetic reactions. CO is present as an impurity in H2 or forms as a stable intermediate in the conversion of small oxygenated organic fuels and is an intermediate in the electro-synthesis of fuels starting from CO2 feedstock. In fuel cells, Pt is the most effective and widely employed catalyst. CO adsorbs strongly to Pt and reduces its catalytic activity. Moreover, CO is a relatively simple molecule and ideally suited for model studies of molecular adsorption at metal/electrolyte interfaces. To deepen understanding of elementary steps involved in electrochemical reactions of CO, the single crystal electrodes Pt(111), Pt(100) and Pt(335) which is Pt(s)-[4(111) x (100)] in step-terrace notation were employed in this project to investigate CO oxidation in acidic media. Single crystal electrodes have surfaces that are structurally well-defined on the atomic level and provide insight into effects of surface structure on electrocatalytic reaction mechanisms.

Methods for the fabrication of single crystal electrodes are discussed. The responses of in-house prepared Pt(111) and Pt(100) electrodes in comparison to literature benchmarks is demonstrated. Application of the electrodes as standards to monitor the stability and cleanliness of electrochemical experiments involving CO oxidation and oxygen reduction is shown.

The oxidation of CO adsorbed near saturation coverage on Pt(111) and Pt(335) electrodes in 0.5 M H2SO4 over the range of potentials between 0.75 V and 0.9 V (versus a reversible hydrogen electrode (RHE) reference) was shown to adhere to a Langmuir-Hinshelwood (LH) model for adsorbed CO electrochemical oxidation. The model assumes fast CO transport and the reaction of homogeneously adsorbed CO and OH on the surface to form CO2 For Pt(100) electrodes, the responses were somewhat more complicated than those predicted by the LH model. Furthermore, introducing defects into Pt(100) by cooling the electrode in an Ar atmosphere (without added H2) following annealing resulted in responses similar to those for CO oxidation over nanometer-scale (< 10 nm diameter) Pt catalyst particles. In particular, current-time transients recorded in potential step measurements showed tailing at long times characteristic of CO oxidation on Pt catalyst particles. The rate of CO oxidation over the Pt single crystal electrodes decreased in the order of Pt(335) > Pt(100) > Pt(111). Additionally, for CO adsorbed to full coverage at 0.1 V (versus RHE) on Pt (335) in 0.5 M H2SO4 at ambient temperature, oxidation of the layer gave 7.6 x 1014 CO/cm2 as the saturation CO coverage, just below the average value reported for CO on Pt(335) in ultra high vacuum (8.3 x 1014 CO/cm2).

The oxidation of CO adsorbed to sub-saturation coverage on Pt(335) was also investigated. Responses for fractional CO monolayer (ML) coverages, determined relative to the Pt(335) surface atom density, were recorded for the range 0.08-0.52 ML, where 0.52 ML is the maximum attainable coverage. For reactions at 0.7 V (versus RHE) in 0.05 M H2SO4, numerically solving the rate equations to the LH model of adsorbed CO electrochemical oxidation reproduced the main features in current-time transients from potential step experiments. Above half saturation ( > 0.26 ML), the transients progressed through a current maximum, whereas below 0.26 ML a continuous decay were observed, in accordance with predictions of the LH model.

Pt(111) and Pt(335) were intentionally contaminated with n-thiol and acetonitrile before adsorbing CO onto their surfaces. CO could displace acetonitrile from both surfaces. However, n-thiol adsorbed strongly to both surfaces and increased their coverages with potential cycles. On both surfaces the effect of contamination was to shift the CO oxidation stripping peaks to higher potential and decrease the CO oxidation rate. These responses are similar to those observed for CO oxidation on Pt nanoparticles, indicating contamination may play a role in CO oxidation on nanoparticle surfaces.

Investigations were extended to determine the effects of adsorbed anions and crystalline defects, generated by applying a potential 1-2 V for 1 s to Pt(100), on CO oxidation. With more strongly adsorbing anions in the supporting electrolyte, CO oxidation became slower. The CO oxidation stripping peak shifted positively in the electrolytes with increasing concentration of more strongly adsorbing anion species. Applying higher anodic potentials to Pt(100) generated greater degrees of defects on the surface. When a small degree of defects was introduced to Pt(100), CO oxidation proceeded faster than on the well-ordered surfaces. However, at the highest levels of defects, the oxidation rate slowed. Based on the hard sphere model, the hydrogen adsorption charge indicates a transformation of Pt(100) to a structure of similar step density as Pt(11,1,1) = Pt(s)-[6(100) x (111)] with applied potential of 2 V for 1 s.

Description

Keywords

Langmuir Hinshelwood Mean Field model, Carbon monoxide (CO) oxidation

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